[Boatanchors] A story of two grounds.
Philip Atchley
[email protected]
Mon, 21 Oct 2002 21:16:01 -0000
Hello all.
Sometime back I re-capped and serviced this '67EAC R-390A that I'm presently
using and it performs VERY well. I chose to go the "under chassis" route
for the 'lytics rather than rebuild the cans.
One thing that I noticed was that this set had some 120Hz noise in the Local
audio that, while not loud was noticable with the volume turned all the way
down. NOT a hum, it was more like the line related switching noise one
hears from Triac controlled lamps etc. So today I decided to troubleshoot
it (I HAVE to get a helper to move this thing around 8^). I suspected a
ground loop.
To make a long story short. When I mounted the capacitors under the chassis
I mounted two solder lugs under existing screws, cleaning under them to make
sure they made good contact. This so I could make a VERY neat installation
of the capacitors as the lugs provided a convenient mounting point for the
ground end of the 'lytics. I've done this before on other R-390A's with
good luck. Previous sets had the Solid state rectifiers while this one has
intact tube rectifiers. Anyway, on a hunch I replaced the ground lugs with
two insulated terminal strips (Radio Shack) in which I had cut off all but
one insulated lug. I then connected the negative end of the capacitors to
these lugs and ran a ground wire back to the two appropriate Capacitor Octal
sockets giving the capacitors their original ground points.
Upon testing, Voila! That did the trick. IF I put my ear on the speaker I
can 'just discern' the noise. In normal operation certainly not noticable
like it was.
MORAL OF THE STORY: I'm sure Art Collins spent a lot of money researching
and designing the grounds in our beloved R-390A's, don't try to second guess
him 8^)
One additional Note: The ground lug where these caps are originally
grounded IS NOT all that far from where one of my ground lugs was located.
73 de Phil KO6BB
Chasing NDB's (Non-Directional Beacons)with .....
LF Modified R-390A,'67 EAC (26 valves).
Homebrew 'Hybrid' Longwave CW receiver.
20 Meter Long Alpha Delta Sloper Antenna.
50dB Gain Low Noise Tunable Preamp.
Heathkit HD-11 Q-Multiplier.
Merced, Central California.
37.18N 120.29W CM97sh.